1. Field of the Invention
An object of the present invention is a device for the exploitation or utilization of electrical signals, especially signals delivered at a high bit rate. The invention is more particularly applicable in the audio-visual field or in the field of data processing. It is aimed at enabling the interposition, in a transmission channel going from a receiver to a signal-utilization device, of an auxilary circuit that plays a role in the processing of the received signals or that replaces these signals.
2. Description of the Prior Art
In the audio-visual field, currently used television sets have few interfaces with the outside world. Most of these television sets have only one high-frequency input (antenna input) that is connected to the television set and gives a signal to be displayed. In Europe, and more especially in France, a connector known as a PERITEL type connector has 21 pins. It gives the fullest possible range of input/output signals available to date, with analog video and audio inputs and outputs in separate basebands (SECAM or PAL or even NTSC encoded video signals). It furthermore provides the possibility of entering a main or superimposable analog RGB signal. Certain manufacturers propose a series data bus called "D2 B" used to transmit man/machine interface commands and data elements. Certain "monitor"-type television sets outside France have video or audio inputs/outputs equivalent to the baseband signals of the PERITEL connector, but these inputs are on an RCA plug.
Known audio-visual systems with access control, for example those used in subscriber television systems, are divided into two functional parts. A first part relates to a system for the scrambling/unscrambling (or descrambling) of the electrical signals conveying the image and/or the sound and/or the associated data elements (for example teletext data elements). (Hereinafter, the terms "unscrambling" and "descrambling" will be used interchangeably, as will the terms "unscamble" and "descramble" and the terms "unscrambler" and "descrambler." This first part converts the signals so as to make them unintelligible to an unauthorized entity. For example, there are known video line shifting systems, video line cut-off and rotation (or rather translation) systems (known as LCAR systems) or even audio spectrum reversal systems. The scrambling is done by the signal transmission source. The unscrambling (which is a function symmetrical to the scrambling function) is carried out at the reception of the signals.
The scrambling at transmission and the unscrambling at reception are kept in synchronism by a determined temporal sequence. For example, this sequence takes the form of a physical signal that is conveyed simultaneously with the scrambled signal. A physical signal such as this represents the scrambling signal used at transmission (for example, it is used to describe the location of the cut-off points for each video line in a line rotation and cut-off system). This same signal is used at reception to control the unscrambling in synchronism with the scrambler. In most of the known methods of implementation, the synchronism sequence is constituted by a sequence of control words sent at regular intervals, for example every second. The frequency, however, may be higher in order to allow for the habits of users who change channels frequently.
A second part of the system relates to an access control function which ensures that the synchronism signal described further above is available only at a receiver that has fulfilled certain conditions such as the prior payment of a fee. The access control system often consists of an enciphering system located at the transmission point which makes the synchronism sequence unintelligible before it is sent through the transmission channel. At reception, the access control system ascertains that the conditions of access (for example the payment of a charged fee) are met. If these conditions have been met, then the system authorizes the deciphering (the operation that is symmetrical with the enciphering) of the synchronism sequence, and gives the sequence in uncoded form to the unscrambling system.
In a known mode of implementation, the function of access control at reception is carried out by a microcircuit card while the function of unscrambling at reception is carried out by an electronic circuit that is closely associated with the other functions of reception of the signals. For example, the standard functions for a subscriber television decoder are distributed in a reception circuit connected to an unscrambling circuit. The unscrambling keys are given by a microcircuit card (chip card or chip carrier). In a mode of implementation such as this, the unscrambling function is located inside a main pack called a decoder. The microcircuit card, which can be plugged into the main pack, fulfills the access control function. As a general rule, the unscrambling function may be public and not contain any secrets. The secrets that protect the system reside in the microcircuit card which carries out the function of deciphering the synchronism sequence in real time and on demand.
At present, in most countries, the connection of access-control systems to television sets is done by series connection to the antenna input of the television set. In Europe, the connection may be made by means of the PERITEL connection through the use of signals encoded in baseband (PAL/SECAM/NTSC or even RGB for decoders working on HDMAC or D2MAC signals). To date, television sets do not have interfaces dedicated to the function of unscrambling and access control.
There also exist peripherals for television sets that are capable of producing images and sound locally (apart from video tape-recorders and videodisk equipment). These are notably games panels and ANTIOPE type sub-titling devices. These devices produce an image digitally by the creation of an image memory (bit-map) stored in a specialized memory internal to the device. This image is converted into video signals by conversion into RGB signals or by conversion into signals encoded in baseband (PAL/SECAM/NTSC). If necessary, this signal may even be modulated in microwave mode for television sets having only one antenna input.
The "ANTIOPE" type devices, used in France, can superimpose their images (for example sub-titles for persons with hearing difficulties) by using the superimposing function available on the PERITEL connector through the analog RGB signals). In other countries, the switching between the signal produced by a device such as this (for example a games terminal) and the antenna signal can be done only by means of a hand-operated mechanical switch, enabling the connection of the microwave modulated signal.
The known interfaces of television sets for interfacing with digital audio-visual signals are unsuited at the present time. Indeed, none of the signal interfaces available in present-day television sets is capable of managing audio-visual signals in digitized form. The interfacing needs of digital audio-visual signals are limited today by the small number of devices capable of producing or processing such digital signals. In the near future, with the advent of so-called all-digital television transmission systems, and the appearance of digital video tape-recorders, this requirement will become vitally important for the avoidance of deterioration, through unnecessary digital/analog conversions, in the quality of the signals processed by devices designed for large-scale consumption. A digital interface is therefore indispensable, in the same way as the so-called Y/C connector became necessary and had to be created when the S-VHS system appeared.
With regard to current subscriber television systems as described here above, the signals exchanged at the interface between the part containing the unscrambler and the access control system comprise:
the enciphered sequence for the synchronization of the scrambling, sent regularly to the access control system; PA1 the same synchronization sequence, in uncoded form, after deciphering, sent by the access control to the unscrambler. PA1 it enables the direct connection, without digital/analog conversion, of synthesis video generators such as sub-titling machines, games terminals and personal computers. PA1 it enables the function of unscrambling digital television signals to be transferred to a detachable element. PA1 a receiver to receive these signals; PA1 an utilization device, notably a display device, to utilize the received signals; PA1 a communications interface with a high bit rate, located functionally between the receiver and the utilization device and capable of transmitting the signals from the receiver to the exploitation device and of transmitting these signals to an auxilary circuit and/or of transmitting similar signals from this auxilary circuit to the utilization device, PA1 wherein said device comprises a generator of alternating clock signals synchronous with the high bit rate, PA1 and wherein the interface comprises an orientation circuit to transmit signals from the receiver to the auxilary circuit for a given half-cycle of this clock signal and to transmit signals from the auxilary circuit to the utilization device for another given half-cycle of this clock signal.
It can be seen then that the synchronization sequence of the unscrambler, in uncoded form, is available at the interface. It could be re-routed in order to be given, for example, to other decoders that do not have a valid access control system available.
In systems where the signals are transmitted in analog mode between the transmitter and the receiver, it is sometimes easier for a would-be pirate to tackle the transmitted signal in order to try and understand its modifications by scrambling. For example, he or she may achieve this goal by physical methods of self-correlation. This is preferable to an attempt to reproduce the synchronization sequence which is often constituted by large numbers of binary digital elements (for example 61 bits and more) that are updated frequently (every second).
However, in current systems, and above all in future systems, where the transmission of the signal will be done digitally (for example in high-definition television or in DAB or digital audio broadcasting), a pirate will no longer be able to tackle the transmitted signal by physical methods such as that of self-correlation. Indeed, these signals will conform to the MPEG standards for moving images. In this case, the constitution of the images is done by integration of the successive images with one another for the definition of a definitive image. In practice, there are no longer any video signals as such that are transmitted.
Consequently, would-be pirates would prefer to set their sights on the synchronism sequence of the unscrambler (or even on the unscrambler itself if it is not public). Indeed, this sequence has a lower frequency than the signal itself, and is therefore easier to distribute if necessary. The extent of the risk incurred with such an architecture can be imagined, especially if the unscrambling function is directly integrated with a device such as a television set. If the synchronism sequence is re-routed to the interface between the television set and the microcircuit card, or if the pirate works on the unscrambler itself, then every such television set that has been installed will come under threat.
The invention proposes a solution to the abovementioned interface problem for systems, notably audio-visual systems with digital transmission. The invention consists, for example, of the definition, in this transmission, of a full-duplex, two-way interface with parallel bus, capable of conveying audio-visual digital signals between a television set and the outside world. This interface is the digital equivalent of the PERITEL connector. The problem to be resolved by such an interface is that of seeing to it that the information bit rates, which are necessarily very high when the system is in digital mode, can be conveyed properly between the receiver and an external (auxilary) circuit and then between the external (auxilary) circuit and the television set in real time. (Hereinafter, the terms "external" and "auxiliary" will be used interchangeably.). The interface resolves two problems simultaneously:
In this detachable element, the unscrambling function is directly associated with the access control function. According to the invention, the audio-visual device comprising such an interface no longer holds any secrets and therefore will not be preyed upon in any way if it is designed to reproduce or record a scrambled signal. Should the unscrambled signal ever be compromised, it could be replaced by the changing of the detachable module connected to the interface without the audio-visual device itself being brought into question. Naturally, this audio-visual device may be a computer or other system, using digital data elements.
The invention shall therefore be described with reference to an audio-visual application such as this, without however restricting its scope thereto.